1. Field of the Invention
This invention relates to an optical scanning apparatus and an image forming apparatus using the same, and particularly is suitable for an image forming apparatus such as a laser beam printer, a digital copying machine or a multifunction printer having an electrophotographic process which is designed to cause a beam emitted from light source means to be reflected and deflected by a polygon mirror as a light deflector, and optically scan a surface to be scanned through a scanning optical system to thereby record image information. Also, the present invention relates to a color image forming apparatus using a plurality of optical scanning apparatuses and comprising a plurality of image bearing members corresponding to respective colors.
2. Related Background Art
Heretofore in the optical scanning apparatus of a laser beam printer or the like, a beam optically modulated in conformity with an image signal and emitted from light source means has been periodically deflected by a light deflector comprising, for example, a rotary polygon mirror, and has been converged into a spot shape on the surface of a photosensitive recording medium (photosensitive drum) by a scanning optical system (fθ lens system) having an fθ characteristic, and has been optically scanned on the surface of the recording medium to thereby effect image recording.
FIG. 13 of the accompanying drawings is a schematic view of the essential portions of a conventional optical scanning apparatus. In FIG. 13, a divergent beam emitted from light source means 91 is made into a substantially parallel beam or a convergent beam by a collimator lens 92, and this beam (quantity of light) is shaped by an aperture stop 93 and enters a cylindrical lens 94 having refractive power only in a sub-scanning direction. Of the beam which has entered the cylindrical lens 94, a part in a main scanning cross section emerges in its intact state, and a part in a sub-scanning cross section converges and is formed as a substantially linear image near the deflecting surface 95a of a light deflector 95 comprising a rotary polygon mirror.
The beam reflected and deflected by the deflecting surface 95a of the light deflector 95 is then directed onto the surface of a photosensitive drum as a surface 98 to be scanned through a scanning optical system (scanning lens) 96 having an fθ characteristic, and the light deflector 95 is rotated in the direction of arrow A to thereby optically scan in the direction of arrow B (main scanning direction) on the surface 98 of the photosensitive drum and effect the recording of image information.
To effect the highly accurate recording of image information in such an optical apparatus, it is necessary that curvature of image field be well corrected over the entire surface to be scanned, that there be a distortion characteristic (fθ characteristic) accompanied by uniformity of speed between an angle of view θ and an image height Y, and that the spot diameter on the surface to be scanned be uniform at each image height. There have heretofore been proposed various optical scanning apparatus or scanning optical systems satisfying such optical characteristics.
Particularly in the case of a color image forming apparatus in which a laser beam is directed from an optical scanning apparatus to four photosensitive members (photosensitive drums) to thereby form latent images thereon, and the images of originals of respective colors, i.e., Y (yellow), M (magenta), C (cyan) and Bk (black) are formed on the surfaces of the respective corresponding photosensitive members, the images of four colors, i.e., Y, M, C and Bk, formed on the surfaces of the respective photosensitive members are superposed on a transferring material such as paper and therefore, if curvature occurs to the scanning line of the optical scanning apparatus corresponding to each photosensitive member, there will arise the problem that an error occurs to the shape of a scanning line among the four colors and color misregister occurs in the images on the transferring material and this results in the remarkable deterioration of image performance.
Several causes of the occurrence of the bending of the scanning line are conceivable, and among them, qualitative description will be made below about a case where the scanning lens is inclinedly mounted.
FIG. 14 of the accompanying drawings is a perspective view of essential portions when a flat glass plate 121 is inclined in the scanning optical system (when it is rotated about an axis orthogonal to the optical axis of the scanning lens and the rotary shaft of the light deflector), FIG. 15 of the accompanying drawings is a cross-sectional view of the essential portions in the main scanning direction in FIG. 14 (main scanning cross-sectional view), and FIG. 16 of the accompanying drawings is a cross-sectional view of the essential portions in the sub-scanning direction in FIG. 14 (sub-scanning cross-sectional view).
When here, the thickness of the flat glass plate 121 is defined as L, and the refractive index of the material thereof is defined as n, and the angle of inclination of the flat glass plate is defined as θ, and the angle of incidence of an off-axis ray Bi in the main scanning direction is defined as φ, an on-axis ray Ai is bent downwardly in the sub-scanning direction by a surface 120, whereafter it emerges in parallelism to an incident light Ai while being shifted by a distance ΔZ1 in the sub-scanning direction. Likewise, the outermost off-axis ray Bi also emerges while being shifted by a distance ΔZ2 in the sub-scanning direction. Assuming here that there is nothing between the flat glass plate 121 and a surface 122 to be scanned, the bending ΔZ of the scanning line on the surface 122 to be scanned is:ΔZ1≈L·sin(θ−a sin(sin θ/n))ΔZ2≈L·sin(θ−a sin(sin θ/n))/[cos(a sin(sin φ/n))]ΔZ≈ΔZ2−ΔZ1=L·sin(θ−a sin(sin θ/n)·[(1/cos(a sin(sin φ/n)).  (Expression A)
Actually, the scanning lens is of a shape curved in the main scanning direction and has power also in the sub-scanning direction and therefore requires a detailed analysis, but when an optical part constituting the scanning optical system is mounted while being rotated about the axis orthogonal to the optical axis of the scanning lens and the rotary shaft of the light deflector, there arises the problem that the bending of the scanning line occurs on the surface to be scanned.